1. Field of the Invention
This invention relates to dimmers, and particularly to improvements in a dimmer suitable for use to control the intensity of illumination of a lamp or lamps for illuminating an instrument panel provided in a compartment of a vehicle such as a motor car, comprising a dimming transistor connected to a lamp or lamps to control an electric current supplied to the lamp or lamps and a switching circuit connected to said dimming transistor for generating pulses having a controllable duty ratio so as to turn `ON` or `OFF` said dimming transistor.
2. Prior Art
It is desirable that the intensity of illumination of the lamp or lamps for illuminating the instrument panel provided in the compartment of the vehicle such as the motor car are suitably varied in accordance with the running condition of the vehicle. More specifically, in the case external illuminations such as neon signs are many as the vehicle runs through urban districts, it is desirable to increase the intensity of illumination of the illuminating lamp or lamps, so that the visibility of instruments in the instrument panel may be increased. On the other hand, in the case external illumination are few as the vehicle runs through suburban districts, it is desirable to decrease the intensity of illumination of the illuminating lamp or lamps, so that the fatique of eyes may be prevented. Consequently, heretofore, for the abovedescribed purpose, there has been practised to insert a sliding resistor into a lamp circuit in series, whereby said resistor is increased or decreased in resistance, so that the intensity of illumination of the lamp can be controlled. However, with this arrangement, such disadvantages have been presented that an electric current to be supplied is high in value, thus resulting in low durability of the dimmer.
On the other hand, in order to reduce the heat generation due to dimming, materialize high reliability and durability and obtain a smooth and comfortable dimming characteristics, there has been proposed a transistor chopper type dimmer as shown in FIG. 1. Referring to FIG. 1, designated at L.sub.1 -L.sub.N are lamps to be dimmed, 10 a dimming transistor connected to said lamps L.sub.1 -L.sub.N for controlling electric currents supplied to said lamps, 12 a transistor chopper type switching circuit connected to said dimming transistor and generating pulses having a controllable duty ratio to turn on or off said dimming transistor 10, +B an electric power terminal connected to a battery not shown, and 14 and 16 noise arresting capacitors provided between the poles of the dimming transistor for preventing noises caused by the switching of the switching circuit 12 from being mixed with radio or the like, in which 14 is the waveshaping capacitor constituting a Miller integrator and 16 is the capacitor for preventing oscillations of a circuit constituted by the lamps L.sub.1 -L.sub.N, capacitor 14 and dimming transistor 10.
As shown in FIG. 1 for example, said switching circuit 12 comprises a non-stable multi-vibrator constituted by transistors 21, 22, capacitors 23, 24, resistors 25, 26, 27, 28 and 29 and variable resistor 30. The duty ratio of the pulses generated by this switching circuit is controllable by the control of the variable resistor 30. Furthermore, the capacitors 23, 24 are choppering capacitors and the resistors 25-29 and variable resistor 30 are choppering resistors.
In the conventional transistor chopper type dimmer as described above, pulses having a preset duty ratio determined by the variable resistor 30 in the switching circuit 12, and the switching of the transistor 10 is controlled by said pulses, with the result that the currents supplied to the lamps L.sub.1 -L.sub.N are controlled, so that the quantity of light generated can be controlled.
In the conventional transistor chopper type dimmer as described above, in order to prevent the noise mixing with a communication equipment such as a radio of the malfunction of an electrical equipment such as an electrically controlled fuel injection device due to the switching, the noise arresting capacitors 14 and 16 are inserted between a base and collector or between a collector and emitter of the transistor 10, as shown in FIG. 1, to lengthen the time periods required for rise or fall of the switching. This leads to the tendency of increasing of the heat generation in the dimming transistor 10. Consequently, in order to prevent this, it is necessary to reduce a voltage V.sub.ceo between the collector and emitter of the transistor 10. However, due to the characteristics of semiconductors, if the voltage V.sub.ceo is reduced, then a breakdown voltage Vces between the collector and emitter is necessitated to be low. Consequently, as frequently experienced with the actual vehicles, when a positive surge voltage, in which the rise time is t.sub.1 =1 .mu.sec-10 msec, the fall time is t.sub.2 =1 msec-several hundred msec and the peak voltage is Vp=several ten-several hundred V as shown in FIG. 2, is applied to the power line through the power terminal +B as shown in FIG. 1, the dimming transistor 10 may be broken down, thus resulting in low reliability of the dimmer.